How does keeping a journal help a writer's practice?

A note from Nava: Sorry for the months of silence! But I have a good excuse; I was writing two books, and the deadline pressure was excruciating as well as exhilarating. One was my next vegan cookbook, but the other was none other than The Literary Ladies' Guide to the Writing Life, the more fleshed-out version of this blog. Stay tuned; it is due out in April, 2011!

Dear Literary Ladies,
Do you think it's a good practice to keep a journal? What did you use your journal for, and how did it benefit your writing practice?

One of the most helpful tools a writer has is [her] journals. Whenever someone asks how to become an author, I suggest keeping a journal. A journal is not a diary, where you record the weather and the engagements of the day. A journal is a notebook in which one can, hopefully, be ontological.

A little more pragmatically, a journal, at least one that is not written for publication, and mine most certainly are not, is a place where you can unload, dump, let go. It is, among other practical things, a safety valve. If I am in the slough of despond, if I am in a rage, if I am, as so often, out of proportion and perspective, then, once I have dumped it all in the journal, I am able to move from subjectivity to at least an approach to objectivity, and my family has been spared one of Madeleine’s excessive moods. A journal is also a place in which joy gets recorded, because joy is too bright a flame in me not to burn if it doesn’t get expressed in words. And it’s where I jot down ideas for stories, descriptions of a face seen on a subway, a sunset seen over the Hudson, or our Litchfield Hills.

—Madeleine L’Engle, A Circle of Quiet, 1972

What